Conventional steam powered plants burning pulverized coal continue to provide a major portion of the nation's electric power generation. Improvements in thermal efficiency of these plants will require the use of steam cycles operating at higher temperatures and pressures than those presently used. Since the materials that are now available are limited in their physical capabilities, steam conditions must be closely controlled.
Research is currently being pursued at Oak Ridge National Laboratory (ORNL) to develop austenitic alloys for use as superheater/reheater tubes in boilers of advanced steam cycle fossil power plants which must be able to withstand temperatures of 650.degree.-700.degree. C. with 35 MPa steam pressure inside. There is a need for alloys with improved strength but without disadvantageous physical properties such as difficulty of fabrication or susceptibility to corrosion. Alloys previously developed, and defined in the Table contained in this application, have drawbacks that need to be overcome. Alloy 617 is a superalloy that could meet strength and corrosion requirements for boiler tubing of an advanced steam-cycle power plant, but since it is high in Cr, Mo, Co and Ni it is about five times more expensive than a typical type 304 316, or 347 austenitic stainless steel. These conventional 300-series steels are employed as boiler tubes in existing fossil power plants today, but their strength and corrosion resistance limit both metal temperature and steam pressure to about 540.degree. C. and 24 MPa, respectively.
Recent research at ORNL for advanced steam cycle boiler materials has produced several "lean" (14Cr) austenitic stainless steels with outstanding creep rupture strength at 700 C., approaching that of alloy 617. This was done using minor alloying element compositional modifications that produce specific precipitate microstructures directly resulting in an improvement in properties. However, despite their creep strength, the lower Cr content of these alloys demands that they be protected against fire- and steam-side corrosion by cladding or chromizing.
The closest conventional higher chromium alloys to those new alloys under development at ORNL are alloys 800 and 800H, which have been the subject of study for use in nuclear applications such as vessel or core components for high temperature gas cooled reactors or liquid metal fast breeder reactors. The alloys, particularly 800H, have also been used in the petrochemical industry due to their good resistance to corrosion. Alloy 800H, with 20Cr--30Ni, is a candidate for use in steam applications since it adequately resists steam-side corrosion; however, its strength at elevated temperatures can be affected by precipitation of .gamma.' [Ni.sub.3 (Al,Ti)] at 500.degree.-650.degree. C. or precipitation of M.sub.23 C.sub.6 and/or MC at higher temperatures. Other efforts to modify 800H for non-nuclear applications such as advanced steam cycle boiler tubes involve the removal of Ti and/or Al and the addition of Mo and Nb.